Existing memories may be classified into volatile memories and nonvolatile storage memories. Examples of volatile memories include static random access memories (SRAMs) and dynamic random access memories (DRAMs), and examples of nonvolatile storage memories include NAND flash memories and hard disk drives (HDDs).
The volatile memories consume large energy due to leakage in the SRAMs, and refresh current in the DRAMs. In order to solve this problem, various nonvolatile working memories that may possibly replace the SRAMs and the DRAMs have been studied.
The working memories, however, are accessed more often in an active state than in a standby state. Since a large writing charge (Qw) is required in the active state, energy required for a write operation increases. As a result, the energy saved during the standby state due to their nonvolatile characteristic is completely consumed in the active state, and therefore the total energy consumption is difficult to be decreased. This is called “nonvolatile memories' historical dilemma.” No existing product has solved this problem.
Recently performed experimental simulation using best data in laboratories may be solving the problem to reduce energy consumption if a STT (Spin Transfer Torque)-MRAM (Magnetic Random Access Memory) is used as a lowest level cache memory (LLC (Last Level Cache)), which is relatively not frequently used.
If the STT-MRAM is used as a cache memory that is higher in level than the LLC, the frequency at which it is accessed considerably increases. Therefore, considerable energy is consumed. The aforementioned energy consumption problem thus may not be solved.